The insurgent safe house in Fallujah looked like every other one on the block, except that it was carefully marked with two new bricks, hanging from cord on the outer wall.

Explosives experts had already been in the carport, and blown up several mortar tubes set up in the back of a truck - a mobile artillery unit favored by rebels.

But Tuesday 1st Sgt. Rodolfo Sarino wanted to take the counterinsurgency effort one step further, in keeping with the volumes of new information US troops are learning every day about the rebellion they are trying to crush.

He upended barrels full of drinking water, and used his knife to carve slits into plastic water jugs, draining them, too. Then the marine examined the large store of food adjacent to the kitchen: sacks of rice, bowls of potatoes, and onions.

Within minutes, the store was on fire, adding belching black smoke to Fallujah's acrid skyline - and depriving mobile bands of insurgents of at least one life-giving larder.

"That's how they move from place to place and survive," Sergeant Sarino said, as flames began licking up the food cache. "They go from house to house, and stockpile food and water wherever they can. You have to [burn it], because that's the only way to defeat them: Deny them food and water, and force them to come out."

Such tactics are paying off to a degree, in a guerrilla fight on an urban battlefield, where the learning curve for conventional US Marine and Army forces has been steep.

Hungry insurgents have been found among the dozens of men who visited food distribution sites tentatively opened by the Iraqi National Guard on Monday.

Those suspects have been questioned, and in one case, have led Americans to a safe house stacked with food, a suicide belt packed with explosives, dozens of electronic garage-door openers (used to set off command-controlled car bombs), and other hardware.

US, insurgents learning more

But the invasion of Fallujah - nerve center and symbol for Iraq's nationwide insurgency - enters its second week, US and Iraqi forces are learning more and more about each other.

Tactics learned here will almost certainly be used against insurgents elsewhere in Iraq, who have taken the Fallujah assault as their cue to attack in a string of cities across Iraq.

US troops Tuesday had to send a 1,200-strong force to retake control of the center of the northern city of Mosul. In Sunni strongholds, guerrillas overran nine police stations. In recent days, they have killed scores of people.

The US is trying to stamp out the insurgency in Sunni regions of Iraq ahead of elections scheduled for January. A Sunni politician who withdrew from Prime Minister Iyad Allawi's government to protest the Fallujah assault was arrested Tuesday in Baghdad by US forces, the Associated Press reported.

Holdouts remain

In Fallujah, US forces now occupy all the city, but they are far from subduing a persistent enemy. While hundreds of insurgents have been killed - among them, officers believe, the majority who were interested in suicide martyr operations - many remain who have proven to be smarter, and don't want to die.

US officers say that insurgent tactics are evolving, just as American ones are too, in response. They say they have found evidence of tunnels connecting houses - a ploy once preferred by Serb snipers in Sarajevo - and even use of the underground sewer system.

Insurgents have fired from behind curtains, to mask the flash of their muzzles. They have used armor-piercing ammunition and advanced sniper rifles.

They have also turned living rooms into machine-gun nests, lying in wait for marines who these days are clearing houses. In recent days, two marines died when they crashed through a front gate to clear a house.

Two more casualties came from another incident, in which insurgents had taken a family hostage. Marines broke in, they say, held their fire because of the family, and then were shot themselves.

Marines now use far more "dynamic" entries, which include using explosives to blow open doors, breaching ladders to scale walls and even to bridge separate houses, and flash-bang and fragmentation grenades to clear rooms.

Some platoons are running short of shotgun shells, because so many doors have been breached with them.

One of the biggest firefights took place when Alpha Company set up a platoon base two doors down from a group of more than 30 insurgents. For two days, the rebels kept their activity hidden and quiet.

"They didn't do a thing until we were on their door," says one officer, about the incident that turned into a noisy gunbattle that killed an estimated 15 insurgents in the eastern section of the city. "They shot, and then ran away. The first chance these guys had to run, they did. They ran down the easiest path."

But finding the safe houses and remaining insurgents has not been easy for the 10,000 US troops in a city of 300,000 residents, nearly all of whom have moved out during hostilities.

Marines say they have found lots of drugs in safe houses, probably amphetamines similar to speed, to keep them awake. Al Qaeda safe houses in Kabul, after the Taliban fell in November 2001, contained such drugs.

"The enemy did not respond, until we knocked on the exact door," says another officer.

Amateur mistakes

But other incidents on this battlefield attest to the amateur nature of many insurgents. In one case, 40 or so insurgents gathered and ran openly down a street, with weapons slung on their backs.

When they hid in a building, US forces didn't hesitate: They destroyed the whole building.

"We've go to get inside their heads," one officer told fellow commanders. "Tactics are evolving, ours and theirs. We've killed those who want to die, and the stupid ones. Now they are smart. And want to live."

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